The present invention relates generally to wireless local audio transmission, and specifically to high quality wireless audio speakers.
Wireless local transmission of audio data has become very popular in recent years. Wireless transmission eliminates the need for wires which are annoying and limiting. Local wireless transmission is used, for example, in portable home telephone systems, stereo systems, home theater systems, and sound surround systems.
U.S. Pat. Nos. 5,272,525, 5,349,386, and 5,319,716, which are incorporated herein by reference, describe wireless local transmission systems which transmit audio and/or video signals as modulated RF signals.
However, wireless transmission systems are limited in their transmission range and quality. One transmission problem is due to interference from other wireless communication systems, such as cordless telephones and remote control devices. These limitations are especially annoying in audio systems for high quality music transmission.
In order to avoid such interference and limitations U.S. Pat. No. 5,491,839 suggests having a transmitter and one or more receivers which allow a user to select a frequency band for transmission. Thus, the user may select a band which has less interference than other bands. In addition, the receivers have a muting circuit which is operated when a faulty signal is encountered in the transmitter or in the receiver.
PCT publication WO97/29550 describes transmitting audio data as digital signals. A forward error correction encoder and an interleaver are used to minimize the damage caused by noise and interference to the audio data.
However, improvement of transmission of audio data is still sought. It is particularly desirable to achieve high-fidelity audio transmission without requiring adjustments to be performed by the user.
It is an object of some aspects of the present invention to provide a system for high-fidelity local transmission of audio data.
It is another object of some aspects of the present invention to provide methods and apparatus for digital transmission of audio data in relatively narrow bands, such as, the ISM 900 MHz band.
It is another object of some aspects of the present invention to provide methods and apparatus for transmission of audio data with two-way control communications.
It is another object of some aspects of the present invention to provide low-cost apparatus for transmission of digital audio data.
It is another object of some aspects of the present invention to provide methods and apparatus for digital transmission of audio data with low transmission delay.
It is another object of some aspects of the present invention to provide systems for transmission of audio data, such that a plurality of systems may be used simultaneously in a single area without interfering each other.
In some aspects of the present invention, the systems, apparatus and methods are used to transmit video data, as well as audio data.
In preferred embodiments of the present invention, audio input data is wirelessly transmitted from a transmitter to one or more compatible receivers. The transmitter and receivers may comprise a standalone system which is used together with separate audio generators and speakers. Preferably, however, the transmitter and receivers are integrated in a stereo or surround system, in which the transmitter is connected to or is a part of a music generator such as a CD or DVD player, and the receivers are connected to, or part of, one or more wireless speakers.
The audio data is converted to digital form, if not already in digital form, and is preferably compressed and encoded to a coded form which allows high-quality, low-delay transmission of the audio data. Preferably, the compression is performed according to a standard compression scheme such as an audio MPEG compression scheme. The audio data is encapsulated together with ancillary data into packets, preferably having a fixed length determined according to transmission and delay considerations, as is known in the art. The methods and apparatus for data encoding and encapsulation, along with the use of the ancillary data, enable the system to convey sound of improved quality, and with greater user convenience, relative to wireless audio systems known in the art.
The ancillary data preferably includes audio control data, such as a volume indication and system control data. The system control data comprises indication of frequencies used and identification information which allows a receiver to easily ignore packets addressed to a different receiver, thus preventing crosstalk. Preferably, the identification includes a unique ID which identifies the transmitting system, and an indication to which receiver of the transmitting system the packet is addressed. For example, the indication to which receiver the packet is addressed may include indication of right or left, front or rear and/or main or sub-woofer.
Preferably, the ancillary data includes indication of a frequency channel at which the current packet is transmitted in order to allow the receiver to tune onto the frequency channel on which the packets are transmitted.
In some preferred embodiments of the present invention, the audio control data comprises a volume level, preferably for right and left receivers separately. Thus, when a low volume is used there is no need to lower the magnitude of the audio input to the transmitter and decrease the dynamic range of the signal. Rather, the audio input is transferred at full magnitude to the receivers which control the volume of the signal according to the volume level.
In some preferred embodiments of the present invention, each packet is transmitted in at least two copies to the receivers, preferably on two respective frequency channels. Preferably, the receivers assess the packets and assign a quantitative performance value to the received packets, generally based on an error detection code, such as CRC. Alternatively or additionally, the signal strength of the received signal is used in assigning the quantitative performance value to the received signal. Preferably, for each packet, the receivers choose the copy having the highest quantitative performance value, and this copy is used by the receiver to extract the audio data.
In one such preferred embodiment of the present invention, when the receiver receives the first copy it calculates the CRC of the received data and compares it to the transmitted CRC. If the calculated CRC is not identical to the transmitted CRC, the first copy is identified as xe2x80x9cdefectivexe2x80x9d, and the second copy is read and its CRC is calculated and compared to its transmitted CRC. If the calculated CRC in the second copy is correct, the audio data of the second copy is played by the speaker associated with the receiver. If, however, the calculated CRC of both copies is defective, then the speaker is preferably kept silent for a predetermined period of between about 0.1 and 1 seconds, preferably using a soft mute procedure described hereinbelow. Alternatively or additionally, the copy with a higher quantitative performance value is played, provided the performance value is above a predetermined threshold. If the calculated CRC in both copies is correct, then the copy with the highest quantitative performance value is chosen.
Alternatively or additionally, the receiver uses the first copy which arrives unless it is found to be defective (i.e., to include noise and/or errors beyond a predetermined level), for example by CRC calculation and comparison. Only when the first copy is defective does the receiver read the second copy and play it if it is not defective.
Preferably, as noted above, the at least two copies are transmitted on at least two respective distinct frequency bands, such that frequency dependent interference may be overcome by choosing the copy of the packet exhibiting a lower level of interference. Further preferably, the at least two frequency bands are separated by a span larger than bands typically used by interfering apparatus, such as cordless telephones. The separating band is preferably of a constant width for any pair of frequencies and is preferably wider than 10 MHz and more preferably about 12 MHz, for systems using bands of a total bandwidth of 26 MHz such as the ISM 900 MHz band. In systems using larger total bandwidths the separating band is preferably larger and is between about 30 and 50% of the size of the total bandwidth used by the system.
Preferably, the transmitter transmits to each operating receiver two continuous streams of packets in the two distinct frequency bands. Each stream of packets preferably includes packets addressed to two separate speakers in an interlaced order. For example, the two speakers may be right and left speakers of a stereo player.
Preferably, the two streams are timed such that at the time the first stream includes a packet addressed to the right speaker, the second stream includes a packet addressed to the left speaker, and vice versa. Such timing prevents a transient interference from destroying both copies of the packet, since the two copies are not transmitted at the same time. In addition, there is no need for any of the speakers to have more than one receiver to receive both copies of a packet from the two different channels. A single receiver may be tuned onto the first channel to receive the first packet and then immediately move to the second channel to receive the second copy on the second channel. Preferably, each packet is transmitted for about four milliseconds and the switching time required to move from one channel to the other is less then 5 xcexcsec. It is noted that prior art systems use frequency diversity in which two copies of a packet are transmitted simultaneously. In these systems two different receivers are needed to receive both copies of the packet.
In some preferred embodiments of the present invention, the at least two distinct frequency channels are chosen from a plurality of predetermined frequency channels, which are preferably organized in predetermined pairs. In a preferred embodiment of the present invention, the system has at least eleven pairs of predetermined channels, which are within a range assigned by the FCC for use by unlicensed users, such as 902-928 MHz, 2.4-2.483 GHz, or the 5.5 GHz range. It is noted that the 902-928 MHz band is relatively narrow and therefore requires use of narrow-band frequency channels to allow for a plurality of unique frequency channels. Preferably, the bandwidth of each of the frequency channels is narrower than 1 MHz and most preferably less than 750 kHz. Further preferably, the transmitter does not use a spread-spectrum method of modulation, but rather concentrates the data in the relatively narrow band.
In some preferred embodiments of the present invention, the frequency channels are chosen from the plurality of predetermined channels automatically by the transmitting unit. Preferably, in systems as described below in which the receivers transmit control signals back to the transmitter, the channels are chosen according to tests performed on a plurality of bands to determine the bands which have low interference rates.
In some preferred embodiments of the present invention, the receivers automatically lock onto the frequency bands of the packets transmitted from the transmitting unit. Preferably, using the ancillary information included in the packets, the receivers automatically tune to the frequency which provides best reception quality of the packets transmitted on the channel. Preferably, the packets include an indication of the frequency on which they are primarily transmitted so that the receiver may verify and/or switch to the primary frequency. The packets are preferably checked for their ID to prevent locking onto a frequency of signals from a different system.
In some preferred embodiments of the present invention, stereo audio signals are transmitted as two separate mono signals, thus allowing independent transmission of the signals, resulting in better quality transmission. Thus, each receiver needs to receive only its own mono signals, which naturally comprise a lower data volume than the stereo signals. Therefore, the transmission time required is reduced, and the time saved may be used for the double transmission of the signals described hereinabove. In other words, instead of having the receivers receive long data packets including the audio information for two receivers, the packets are divided into two parts, and each receiver may receive to the part which it needs.
In some preferred embodiments of the present invention, the transmitter and/or receivers include a user interface which allows the user to monitor and modify the transmission in addition to the automatic monitoring functions described herein. Preferably, the user interface (typically of the receiver) includes a display, such as an LED bar graph, indicative of the signal quality, and controls which allow adjustment of the acquisition of the signals and of the identity of the speakers (left/right). Preferably, the signal quality is a multi-level function of the percentage of packets in which errors are detected and of the received signal strength.
The user interface of the transmitter preferably includes a control which initiates changing one or more of the frequency channels that are in use. In addition, the user interface of the transmitter preferably allows adjustment by the user of other features such as the volume, ID generation and power on/off control.
Alternatively or additionally, two-way control communication may be implemented, as described hereinbelow, between the transmitter and receivers, in which case each of the elements of the user interface may be implemented in either the transmitter, or in one or more of the receivers, or in both the transmitter and the receivers (i.e., the speakers). Such flexibility in the location of the user interface makes it easier for the user to position and adjust the speakers for optimal audio reception.
In some preferred embodiments of the present invention, the receivers transmit control signals back to the transmitter to monitor the transmission. Preferably, when a high occurrence rate of defective packets is detected at the receiver in a particular frequency channel, the receiver automatically sends a request to the transmitter that the packets be transmitted using a different frequency channel.
Preferably, major functions of the transmitter and receiver, such as compression and decompression, and operations conventionally performed by a modem, or a controller, are implemented mainly in software so as to reduce the cost of the hardware and to achieve greater flexibility in the operation of the system.
There is therefore provided in accordance with a preferred embodiment of the present invention, a wireless transmission system for transmitting audio data, including a transmitter which re-frames the audio data into a plurality of packets and wirelessly transmits at least two copies of each packet during substantially non-overlapping periods, and one or more receivers which receive the plurality of packets and assess a quality level of at least one of the copies thereof, so as to extract the audio data from one of the copies having a desired quality level.
Preferably, the transmitter compresses the data using a standard compression method.
Preferably, the transmitter re-frames the data by adding a correction code to the audio data.
Preferably, the audio data includes stereo signals, and the transmitter compresses and transmits the signals as a pair of mono signals.
Preferably, each packet includes audio signals belonging to only one of the pair of mono signals.
Preferably, the transmitter adds a volume level indication to at least some of the packets, so that the audio data has a generally constant magnitude irrespective of the volume level.
Preferably, the transmitter transmits the at least two copies on at least two respective, distinct carrier frequencies.
Further preferably, the distinct frequencies are mutually separated by at least 10 MHz.
Preferably, re-framing the data includes adding an indication the frequency at which the packets are transmitted.
Preferably, a user initiates a change of the frequencies by inputting a user signal to the transmitter.
Preferably, the receivers automatically tune in to the distinct frequencies.
Preferably, the transmitter transmits an indication of the distinct frequencies along with the signals.
In a preferred embodiment, the frequencies are in the band 902-928 MHz.
Preferably, the plurality of packets are transmitted in a signal having a bandwidth narrower than 1 MHz.
Preferably, each receiver assesses the quality of the at least two copies of the packets and extracts the audio data from the copy which is of better quality.
Alternatively or additionally, the receiver assesses the quality of a second copy of the at least one copies if a first copy of the at least one copies has an undesired quality.
Preferably, when the quality level of substantially all of the copies of a packet is beneath a predetermined quality threshold, the audio data in the packet is discarded by the receiver.
Preferably, the receiver plays silence instead of the discarded audio data.
Preferably, a copy is beneath the predetermined quality threshold when an error is detected in the copy.
Preferably, the transmitter adds an ID to the packets, and the receivers ignore packets which do not carry the correct ID.
Preferably, the receivers include back-transmitters which transmit control commands back to the transmitter.
There is further provided in accordance with a preferred embodiment of the present invention, wireless speaker system for audio data, including a transmitter which wirelessly transmits the audio data, and at least one speaker, which receives and plays the audio data and which includes a back-transmitter, which transmits control commands back to the transmitter responsive to the data.
Preferably, the back-transmitter transmits an indication of the quality of the received packets and the transmitter changes a parameter of the audio transmission accordingly.
Preferably, the transmitter changes the frequency of transmission responsive to an indication of low quality of the received audio data.
There is further provided in accordance with a preferred embodiment of the present invention, a wireless speaker system for audio data, including a transmitter which wirelessly transmits the audio data on at least one carrier frequency, and at least one speaker, which automatically tunes in to the carrier frequency and plays the audio data.
Preferably, the transmitter re-frames the audio data in a packet including tuning information; and the speaker automatically tunes in to the carrier frequency using the tuning information.
Preferably, the tuning information includes an identification of the carrier frequency, an identification code of the at least one speaker, and/or an error detection code.
There is further provided in accordance with a preferred embodiment of the present invention, a method of wirelessly transmitting digital audio data, including re-framing the audio data, transmitting at least two copies of the re-framed audio data during substantially non-overlapping periods, receiving the at least two copies, assessing a quality level of one or more of the at least two copies, and extracting the audio data from one of the copies which has a desired quality level.
Preferably, transmitting the at least two copies includes transmitting the at least two copies on at least two respective, distinct carrier frequencies.
Preferably, transmitting on the at least two respective, distinct carrier frequencies includes transmitting on at least two distinct frequencies which are mutually separated by at least 10 MHz.
Preferably, re-framing the data includes adding an indication of the frequency at which the copies are transmitted.
Preferably, receiving the at least two copies includes automatically tuning the receivers in to the distinct frequencies.
Preferably, assessing the quality includes determining an error rate and an AGC level of the received copies.
Alternatively or additionally, assessing the quality includes determining whether errors have occurred in the at least two copies, and extracting the data includes discarding the at least two copies when errors have occurred in substantially all of the at least two copies.
Preferably, re-framing the audio data includes annexing a volume indication to the re-framed audio data.
Preferably, re-framing, transmitting, receiving, assessing and extracting the audio data introduce a delay of less than 50 msec.
There is further provided in accordance with a preferred embodiment of the present invention, a method of wirelessly transmitting digital audio data, including re-framing the audio data into packets which do not carry volume data, annexing a volume level to at least some of the packets, and transmitting the packets.
Preferably, the method includes receiving the packets and playing the audio data at a volume responsive to the volume level.
There is further provided in accordance with a preferred embodiment of the present invention, a wireless audio speaker for playing audio signals transmitted by a transmitter, including a receiver circuit which receives the audio signals, calculating circuitry which calculates a multi-level quality indication responsive to the received audio signals, and a display, which displays the quality indication.
Preferably, the multi-level indication includes at least three indication levels.
Alternatively or additionally, the quality indication is calculated based on an error rate in the audio signals.
Preferably, the audio signals include digital audio signals, and the error rate is determined responsive to a CRC code.
Preferably, the receiver circuit instructs the transmitter to change at least one transmission parameter of the signals when the quality indication is below a predetermined level.
There is further provided in accordance with a preferred embodiment of the present invention, a method of providing an audio signal quality indication for a wireless speaker system, including determining the number of errors in the audio signal, determining the signal strength of the audio signal, rating the quality of the audio signal with a multi-level quality indication based on the signal strength and the number of errors.
Preferably, the multi-level indication includes at least five indication levels.
There is further provided in accordance with a preferred embodiment of the present invention, a receiver of wireless audio signals including a demodulator, which separates the signals at the intermediate frequency into two base-band signal components substantially in phase quadrature, which have two respective DC offsets, a local oscillator (LO) which provides one or more reference signals to the demodulator, and a phase shifter which adjusts the reference signals so as to minimize a difference between the two DC offsets and a desired offset value.
Preferably, the phase shifter adjusts the reference signals so that the two DC offsets are substantially evenly distanced from the desired value.
There is further provided in accordance with a preferred embodiment of the present invention, a wireless transmission system for transmitting audio data, including a transmitter which re-frames the audio data into a plurality of packets and wirelessly transmits the packets, and one or more receivers which receive the plurality of packets, determine whether the transmitted packets are defective, and convey non-defective packets to be played by a speaker.
Preferably, the transmitter transmits at least two copies of each of the packets, and defective packets include packets of which substantially all copies are defective.
Preferably, after receiving a first predefined number of defective packets, the one or more receivers discard a second predefined number of non-defective packets before resuming conveying non-defective packets to the speaker.
Preferably, the first predefined number is between 1 and 3.
Preferably, the second predefined number is between 25 and 32.
Preferably, a third predefined number of packets received after the second predefined number of packets are played at a gradually increasing volume.
There is further provided in accordance with a preferred embodiment of the present invention, a method of sounding wirelessly transmitted digital audio data, including receiving a packet of audio data, detecting an error in the received packet, and playing the packet of audio data if no error has been detected, and playing silence or a previous packet of audio data if the error has been detected.
Preferably, receiving the packet includes receiving two or more copies of the packet, and detecting the error includes determining whether errors occurred in substantially all of the copies.
Preferably, detecting the error includes calculating a value of an error detection code and comparing the calculated value to a transmitted value of the error detection code.
Preferably, the method includes playing silence for a predetermined period after an error has been detected.
There is further provided in accordance with a preferred embodiment of the present invention, a receiver of wireless audio signals which are compressed and encoded, including a down-converter which receives and down-converts a carrier signal carrying the compressed and encoded audio signals, and a single signal processor which decodes and decompresses the down-converted audio signals and controls the operation of the down converter.
Preferably, the signal processor also performs AGC detection and offset determination.
Preferably, the receiver introduces a delay of less than 50 msec between receiving and decompressing the audio signals.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: